Recently, Multiple Input Multiple Output (MIMO) systems have been spot-lighted as the broadband wireless mobile communication technology. Herein, the MIMO system refers to a system, which is used for enhancing communication efficiency by using multiple antennas.
Depending upon whether or not identical data are being transmitted, the MIMO system may be operated by using a spatial multiplexing scheme or a spatial diversity scheme. Herein, the spatial multiplexing scheme refers to a data transmission method (or scheme) enabling data to be transmitted at a fast rate without having to increase the bandwidth of the system, by allowing different sets of data to be transmitted through multiple transmission antennas. The spatial diversity scheme refers to a data transmission method (or scheme) transmitting the same data from multiple transmission antennas, so as to gain transmission diversity. An example of such spatial diversity scheme is Space Time Channel coding.
Additionally, depending upon the number of user equipments being allocated to the same time/frequency area, the MIMO system may be divided into an SU-MIMO (Single User MIMO) and an MU-MIMO (Multi User MIMO). An interval including the time/frequency area may also be referred to a resource area. In the SU-MIMO, one user equipment may be allocated to one resource area. And, in the MU-MIMO, multiple users may be allocated to one resource area. Generally, the performance of the SU-MIMO is more efficient, when the number of users is small. And, the performance of the MU-MIMO is more efficient, when the number of users is large.
However, a wireless channel has non-ideal characteristics, such as path loss, noise, fading effect caused by a multipath, ISI (Inter-Symbol Interference), or a Doppler effect caused by the mobility of the user equipment. Accordingly, diverse techniques are being researched and developed, in order to overcome the above-mentioned non-ideal characteristics and to elevate reliability in mobile communication. Depending upon the presence or absence of channel information feedback from each receiving end to each transmitting end, each of the SU-MIMO and the MU-MIMO may be differentiated as an open-loop mode and a closed-loop mode. The open-loop mode may include an STIC (Space-Time Trellis Code) method, wherein the transmitting end transmits information in parallel, and wherein the receiving end detects the transmitted signal by repeatedly using a ZF (Zero Forcing) method and an MMSE (Minimum Mean Square Error) method, and wherein the receiving end then uses a BLAST region, which can increase the information amount as much as the number of transmission antennas, and a space region (or area), so as to acquire transmission diversity and coding gain. Additionally, the closed-loop method corresponds to a method, wherein the receiving end first estimates the status of the radio channel and, then, transmits the estimated channel status as adequate feedback information to the transmitting end, and wherein the transmitting end controls the channel quality based on the channel status information obtained from the feedback information. Techniques for elevating the reliability in wireless communication may include AMC (Advanced Modulation and Coding). A wireless communication system may use a CQI (channel quality indicator) in order to support the AMC. Herein, the CQI corresponds to information respective to the channel status between the base station and the user equipment. The base station may use the CQI, which is received from the user equipment, in order to decide an MCS (Modulation and Coding Scheme) which is used to perform transmission. By using the CQI, when the channel status is determined to be good, the base station may increase a modulation order or may increase the coding rate, so as to increase the transmission rate. And, by using the CQI, when the channel status is determined to be poor, the base station may decrease a modulation order or may decrease the coding rate, so as to decrease the transmission rate. Once the transmission rate is decreased, the reception error occurrence rate may also be decreased.
Generally, user equipments performing MU-MIMO in the same resource area operate in a group. However, due to the channel interference between the user equipments, the channel performance may be degraded. Therefore, a method for reducing the channel interference between user equipments performing MIMO in the same resource area is required to be researched and developed.